Field of the Invention
The invention relates to a process for producing a casting ceramic including water glass and a powder containing silicon and aluminum.
Casting ceramics are used in the manufacture of electrical insulation. However, they are also suitable for the production of components such as housings and switches. The casting ceramic is a low-viscosity mixture of a highly alkaline water glass phase containing potassium or sodium and an aluminosilicate powder mixture which contains silicon and aluminum in a ratio between 2 and 5. The casting ceramic sets in a polycondensation reaction at temperatures below 100.degree. C. over a period of 2 to 6 hours to give yield a solid body. In the setting reaction, up to 20% by weight of water is formed, and that has to be removed in a drying process at a maximum of 200.degree. C. The casting ceramic is inorganic and can be exposed long-term to high temperatures, after curing and drying. However, a disadvantage of drying is that the water being formed leaves behind it a continuous open porosity of up to 30%. In a moist environment, water can be taken up in the open pores as a result of capillary action and the hygroscopic properties of the surface, which the casting ceramic has, and that leads to impairment of the insulation properties. The continuous porosity reduces the dielectric strength of the casting ceramic which reduces its properties as an insulator.
In German Published, Non-Prosecuted Application DE 41 10 223 A1, it is proposed that the pores be filled with a liquid which is subsequently cured, in order to eliminate the porosity. That process is relatively complicated and can only be carried out with difficulty for large components.